Wet strong tissue paper

ABSTRACT

Tissue paper having a bulk between 2 and 8 g/m 3  and containing an amount of a wet strength agent, wherein the tissue paper contains a wet strength agent in the form of a nitrogen-containing polymer having hydrophobic side-chains. Said tissue paper before converting has a relative wet strength value (RWS) of at least 45%.

[0001] The invention relates to a tissue paper having a bulk between 2and 8 cm³/g, said tissue paper containing an amount of a wet strengthagent and having a wet strength index of at least 1 Nm/g.

BACKGROUND

[0002] In the papermaking art, wet strength agents likeepichlorohydrin-based resins, for example polyaminoamide epichlorohydrinresins have been used for a long time to enhance the strength of paper.Such resins are disclosed in U.S. Pat. Nos. 3,700,623 and 3,772,076. Thewet strength of a paper relates to its ability to maintain physicalintegrity and to resist tearing, bursting, and shredding under use,especially under wet conditions. A further important property of wetstrengthened paper is the softness, especially for tissue paper or thelike. The softness can be described as the tactile sensation perceivedwhen holding or rubbing a paper across the skin. U.S. Pat. No. 5,200,036discloses a wet strength agent which provides paper with enhanced wetstrength. A cationic polyaminoamide epichlorohydrin resin is modified byintroduction of a polymerisable unsaturated hydrocarbon moiety thusproviding it with ethylenically unsaturated side-chain substituents. Theresin is then added to latex-forming monomers whereby co-polymerisationoccurs forming bonds between unsaturated polymerisable hydrocarbonmoieties of the resin and the latex-forming monomers. The reaction maybe assisted by addition of an emulsifier to obtain a desirablesuspension of the formed latex panicles. Resins of the above-mentionedtypes are also used as emulsifiers. Usually, the resins are noteffective enough when used as a sole emulsifier and these are thus usedin combination with an additional compound.

[0003] U.S. Pat. No. 5,314,721 discloses a process for preparation ofvinyl polymer dispersions comprising resin based on a cationicpolyaminoamide whose terminal groups have been substituted withlong-chain aliphatic hydrocarbon radicals which have at least 7 carbonatoms and are derived from monocarboxylic acids. The product obtained isused as a sizing agent.

[0004] U.S. Pat. No. 4,416,729 discloses a method for preparing wetstrength additives comprising the steps of contacting a linearpolyamidoamine with an αβ-ethylenically unsaturated carboxylic compoundto form a substituted polyamidoamine, contacting the substitutedpolyamidoamine with a polyamine to form a branched polyamidoaminebearing a pendant amine moiety, and contacting the branchedpolyamidoamine with an epihalohydrin to form pendant curable ammoniummoieties on the branched polyamidoamine. U.S. Pat. No. 4,416,729 doesnot disclose use of the prepared wet strength additives for productionof tissue paper.

[0005] Although the above epichlorohydrin-based resins in someapplications show adequate wet strength and emulsifying properties, itwould be desirable to be able to provide further and improved wetstrength agents for paper and methods for providing such agents. Itwould also be desirable to be able to provide wet strength resins andagents exhibiting improved softness properties. Further, it would bedesirable to be able to provide further resins having improvedemulsifying properties.

THE INVENTION

[0006] According to the present invention, it has been found thatfurther and improved wet strength agents for paper can be obtained by acomposition containing polymeric particles and hydrophobic hydrocarbongroups providing side-chain substituents on wet strength resins. It hasalso been found a new method for the production of such wet strengthresins and agents. It has further been discovered that the wet strengthagents and resins produced by the method according to the presentinvention give paper improved softness properties without negativelyaffecting the absorbency properties.

[0007] More specifically, the invention relates to paper wet strengthagents comprising polymeric particles and wet strength resins comprisinga cationic nitrogen-containing polymer having hydrophobic side-chainsubstituent. The invention further relates to a method for theproduction of a paper wet strength agent comprising a first step ofreacting a nitrogen-containing polymer with a hydrophobic compound toprovide a nitrogen-containing polymer with hydrophobic side-chainsubstituents, a second stop of reacting the product obtained with acrosslinker to form a cationic wet strength resin and a third stepcomprising emulsion polymerisation of one or more ethylenicallyunsaturated monomers in the presence of the wet strength resin formed.Further, the invention relates to a paper wet strength agent obtainablefrom the method above. The invention further relates to a new wetstrength resin and a method for preparing a wet strength resin accordingto the two first steps as described herein. The invention also relatesto the production of paper comprising addition of a paper wet strengthresin or agent to a cellulosic suspension and to the use of a paper wetstrength resin or agent for the production of paper. The invention alsorelates to papar comprising paper wet strength resins and agents. Theinvention is further defined in the appended claims.

[0008] The present invention provides resins and agents having theability to impart improved wet strength properties to paper. Theinvention further provides a simple, convenient and effective syntheticroute for the preparation of wet strength resins and agents. Thereby,the wet strength resins and agents of this invention can be prepared inhigh yield.

[0009] The present invention also provides wet strength resins andagents which make it possible to produce paper having enhanced softnessproperties. The softness of a paper sheet can be estimated by means ofthe relative wet strength value, which is defined as the ratio betweenthe wet tensile index and the dry tensile index according to the formulaRWS (in %)=(WS/DS)·100 where RWS stands for the relative wet strength,WS is the wet tensile index and DS is the dry tensile index of a paper.RWS is hence a measure of the softness of a paper; the higher the RWS,the higher the softness of the paper. The present wet strength resinsand agents also provide improved emulsifying properties and can be usedas sole emulsifiers without additional compounds which may give rise toundesirable foam formation.

[0010] The term “wet strength agent”, as used herein, refers to an agentcapable of imparting better wet strength properties to paper compared topaper containing no such agent. The wet strength agent comprises a wetstrength resin. The term “wet strength resin”, as used herein, refers toa resin capable of imparting better wet strength properties to papercompared to paper containing no such resin.

[0011] The method for the production of a paper wet strength agentcomprises a first step of reacting a nitrogen-containing polymer with ahydrophobic compound to provide a nitrogen-containing polymer withhydrophobic side-chain substituents, a second step of reacting theproduct obtained with a crosslinker to form a wet strength resin, and athird step comprising forming of particles by emulsion polymerisation ofone or more ethylenically unsaturated monomers in the presence of thewet strength resin formed. According to a preferred embodiment, nopolyamine having at least 2 secondary and/or primary amino moieties,added between the first and the second step, or after the second step,is reacted.

[0012] Suitably, the nitrogen-containing polymer is a polyaminoamide, apolyamine or other nitrogen-containing polymer. Preferably, apolyaminoamide is used which may constitute the reaction product of apolycarboxylic acid, suitably a dicarboxylic acid, and a polyamine. Bythe term “carboxylic acid” is meant to include carboxylic derivativessuch as anhydrides and esters. Suitable polycarboxylic acid includesaturated or unsaturated allphatic or aromatic dicarboxylic acids.Preferably, the polycarboxylic acids contain less than 10 carbon atoms.Suitable polycarboxylic acids include oxalic acid, malonic acid,succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acidand derivatives thereof. Mixtures of these compounds can also beapplied. Suitable polyamines include polyalkylene polyamine, e.gdiethylenetriamine, triethylenetetramine, tetraethylenepentamine,dipropylenetriamine and the like or mixtures thereof. Further, anypolyaminoamide prepared according to a method disclosed in EP 802 215A1, hereby incorporated by reference, may be used. Suitably, themolecular weight of the nitrogen-containing compound ranges from 100 to50000, preferably 500 to 10000. Suitably, the polyamine topolycarboxylic acid ratio is 0.49:1 to 1.49:1, preferably less than1.3:1, e.g. 1.3:1 to 0.7:1. Suitably, diethylenetriamine and adipic acidare reacted to form a polyaminoamide.

[0013] Suitably, the hydrophobic compounds used can contain groups ofcarboxylates or derivatives thereof. The hydrophobation reaction betweenthe nitrogen-containing polymer and the hydrophobic compound can beperformed via alkylation, vinylog addition or other reaction. Thevinylog addition may be illustrated by the following schematic reaction:

[0014] wherein VVV—NH—VVV represents a section of thenitrogen-containing polymer, C═C—COOR represents a hydrophobic compoundcontaining a vinyl group. The vinyl group, i.e. the C═C group, of thehydrophobic compound can react with the nitrogen atoms of the polymer, Rstands for a hydrophobic group of the hydrophobic compound which may bean alkyl, alkenyl, aryl, cycloalkyl or cycloalkenyl group. In case thevinylog reaction is applicable, the unsaturation of the vinyl group ofthe hydrophobic compound is turned saturated after having reacted with anitrogen atom of the polymer.

[0015] According to one preferred embodiment, the hydrophobic compoundis a saturated compound, or an unsaturated compound, resulting in anitrogen-containing polymer having saturated or unsaturated side-chainsubstituents.

[0016] The hydrophobic compounds can contain a hydrophobic groupcontaining up to 40 carbons, preferably 6-40 carbons, and mostpreferably 8-40 carbons.

[0017] The hydrophobic chains of the hydrophobic compounds can beattached to the nitrogen-containing polymer via a chain of atoms, whichcan contain at least one hetero atom, via a covalent bond.

[0018] The hydrophobic compound may be selected from (meth)acrylates,alkenyl(meth)acrylate, alkyl(meth)acrylamides, esters, ethers, diazocompounds, carboxylic acids, acid anhydrides epoxides, alkylsulphonates, alkyl sulphates and mixtures or derivatives thereofcontaining a hydrophobic group, preferably from alkyl(meth)acrylates,alkyl(meth)acrylamides, alkyl sulphonates, alkyl sulphates, diazocompounds, ethers, or epoxides or mixtures thereof, and most preferablyfrom alkyl(meth)acrylates, alkyl(meth)acrylamides or mixtures thereof.Examples suitably include α, β-unsaturated esters or amides like laurylacrylate, 2-ethylhexyl acrylate, dodecyl acrylate,N-alkyl(metha)acrylamides, N-alkylaminoalkyl(meth)acrylamides,N,N-dialkylaminoalkyl(meth)acrylamides,N-alkylaminoalkyl(meth)acrylates, N,N-dialkylaminoalkyl(meth)acrylates,hexyl chloride, 2-ethylhexyl chloride, octyl chloride, decyl chloride,dodecyl chloride, hexadecyl chloride, octadecyl chloride, ethyl epoxide,propyl epoxide, (n-, t-, l-) butyl epoxides, pentyl epoxide, hexylepoxide, 2-ethyl-hexyl epoxide, octyl epoxide, decyl epoxide, dodecylepoxide, hexadecyl epoxide, octadecyl epoxide, hexene, 2-ethyl-hexyene,octene, decene, dodecene, hexadecene, and octadecene.

[0019] The reaction is suitably carried out in water, neat or in othersolvent, e.g. in an organic solvent, e.g. methanol, ethanol, ethyleneglycol or the like, capable of at least partly dissolving the reactantswithout taking part in the reaction under the reaction conditions.Mixture of such solvents can also be used. The reaction is preferablycarried out in water. The molar ratio nitrogen-containing polymer (basedon amino mols) to hydrophobic compound can be at least 1:1, suitably 2:1to 99:1, preferably 3:1 to 40:1. The reaction temperature may range fromabout 25° C. to about 150° C., preferably from about 60 to about 90°C.

[0020] In a second step, the hydrophobised nitrogen-containing polymersare reacted with a crosslinker. The term crosslinker or crosslinkingagent, as used herein, is meant to denote a compound having the abilityto crosslink the resin and/or to form bonds to cellulosic fibres.Suitably, the crosslinkers, sometimes referred to as Intralinkers in EP802 215 A1, describing various intralinkers, hereby incorporated byreference, can comprise epihalohydrins e.g. epichlorohydrin; diepoxides,diacrylates, dimethacrylates, diacrylamides, and dimethacrylamides andmixtures or derivatives thereof are used. Preferably, epichlorohydrin isused as crosslinker.

[0021] The reaction is suitably carried out in an aqueous solution, heator by use of other solvent than water, e.g. ethanol, propanol or thelike or mixtures thereof. Suitably, the solvent can not react with thereactants under the reaction conditions used. Preferably, the reactionis carried out in water. The reaction temperature may range from about0° C. to about 150° C., preferably between from about 4 to about 80° C.The molar ratio of the hydrophobised nitrogen-containing polymer (basedon amino-mols) to crosslinker in the reactant composition may be 10:1 to1:10, preferably 2:1 to 1:2. In a third step according to the invention,the method comprises emulsion polymerisation of one or moreethylenically unsaturated monomers in the presence of the wet strengthresin as formed after the second step herein. The monomers may beselected from styrene, butadiene, vinyl acetate, vinyl amide,alkyl(meth)acrylamide, alkyl(meth)acrylate, e.g. methyl (meth)acrylate,butyl (meth)acrylate, butyl glycidyl(meth)acrylate,2-ethylhexyl(meth)acrylate, dodecyl(meth)acrylate,octadecyl(meth)acrylate; (meth)acrylonitrile, isoprene, or 1,6-hexandioldiacrylate, or mixtures or derivatives thereof. As a result of thepolymerisation process, the formed wet strength resin can be anchored tothe polymeric particles formed yielding a wet strength agent. Asinitiator of the polymerisation reaction, any conventional initiator canbe used. For example, Wako VA 044 can be used. Preferably, the initiatoris water soluble. In the emulsion polymerisation reaction, the wetstrength resin works as an emulsifier during the particle formation. Theformed particle maybe composed of one sole or a mixture of unsaturatedethylenically polymerisable monomers as above exemplified. The reactionis preferably carried out in water, organic solvents, e.g. ethanol,propanol or the like, or mixtures of organic solvents or mixtures ofboth water and organic solvents. The reaction temperature may range from4° C. to about 150° C., preferably from about 30 to about 90° C. Theweight ratio resin to monomer can be 100:1 to 1:100, suitably 10:1 to1:50.

[0022] The invention further relates to a method for preparing a wetstrength resin comprising the first and second steps of the method asabove described.

[0023] The invention also relates to a wet strength agent comprisingpolymeric particles and a wet strength resin comprising a cationicnitrogen-containing polymer having saturated or unsaturated hydrophobicside-chain substituents and a derivative of a crosslinker.

[0024] The polymeric particles can be formed from polymerised monomersas described above. Preferably, monomers are selected from styrene,acrylates and mixtures or derivatives thereof.

[0025] The cationic nitrogen-containing polymer has saturated orunsaturated hydrophobic side-chain substituents and derivatives of acrosslinker attached to the nitrogen atoms of the polymer.

[0026] Examples of suitable nitrogen-containing polymers includewell-known available commercial products which may be prepared asdescribed above or according to conventional methods known in the art.Examples of suitable nitrogen-containing polymers includepolyaminoamides alkyl polyamines, polyamines, and polyvinylamines.

[0027] Hydrophobic saturated or unsaturated side-chain substituents areattached to the nitrogen atoms of the nitrogen-containing polymer. Theterm hydrophobic side-chain substituent is here meant to includehydrophobic groups containing e.g. hydrophobic linear or branchedhydrocarbon chains which can be linked, e.g. via a hetero atom by acovalent bond, to a nitrogen atom of the nitrogen-containing polymer.Hydrophobic groups may also include cyclic chains including cyclichydrocarbons. Combinations of linear, branched and cyclic hydrocarbonsare also included in the concept of hydrophobic groups.

[0028] The hydrophobic group of the hydrophobic side-chain can containup to 40 carbon atoms, preferably 6-40 carbon atoms, and most preferably8-40 carbon atoms.

[0029] The hydrophobic side-chain substituents may derive from e.g.alkyl(meth)acrylates, alkyl(meth)acrylamides, esters, ethers, diazocompounds, carboxylic acids, acid anhydrides, epoxides, alkylsulphonates, or alkyl sulphates, or mixtures thereof containing ahydrophobic group, preferably from alkyl(meth)acrylates,alkyl(meth)acrylamides, alkyl sulphonates, alkyl sulphates, diazocompounds, ethers, or epoxides or mixtures thereof, and most preferablyfrom alkyl(meth)acrylates, alkyl(meth)acrylamides or mixtures thereof.

[0030] Specific examples include substituent derived from α,β-unsaturated esters or amides like lauryl acrylate, 2-ethylhexylacrylate, dodecyl acrylate, N-alkyl(metha)acrylamides,N-alkylaminoalkyl(meth)acrylamide, N,N-dialkylaminoalkyl(meth)acrylamides, N-alkylaminoalkyl(meth)acrylates,N,N-dialkylaminoalkyl(meth)acrylates, alkyl sulphonate, alkyl sulphates,hexyl chloride, 2-ethylhexyl chloride, octyl chloride, decyl chloride,dodecyl chloride, hexadecyl chloride, octadecyl chloride, ethyl epoxide,propyl epoxide, (n-, t -, l-) butyl epoxide, pentyl epoxide, hexylepoxide, 2-ethylhexyl epoxide, octyl epoxide, decyl epoxide, dodecylepoxide, hexadecyl epoxide, octadecyl epoxide, hexene, 2-ethyl-hexylene,octene, decene, dodecene, hexadecene, and octadecene.

[0031] Other suitable substituents may derive from substituted succinicanhydrides containing a group selected from alkyl, alkenyl, aralkyl, oraralkenyl, and ketene dimers or multimers. Further examples of suitablesubstituents may be derived from the compounds disclosed in WO98/39376,hereby incorporated by reference.

[0032] A derivative of a crosslinker can be attached to thenitrogen-containing polymer which makes it possible to create bonds tonitrogen-containing polymers and/or cellulosic fibres. Derivatives of acrosslinker can be derived from epihalohydrins e.g. epichlorohydrin,diepoxides, diacrylates, dimethacrylates, diacrylamides, anddimethacrylamides or mixtures or derivatives thereof may be used.Preferably, the crosslinker is derived from epichlorohydrin.

[0033] According to one preferred embodiment, the cationicnitrogen-containing polymer is either a polyaminoamide-epichlorohydrinresin or a polyamine-epichlorohydrin resin having saturated hydrophobicside-chains. Suitably, at least 10% and preferably up to about 100% ofthe nitrogen atoms of the cationic resin comprise cationic groups.Suitably, up to 100% of the nitrogen atoms of the resin comprisehydrophobic groups, preferably up to 50%, most preferably 5-30%.Suitably, the wet strength agent comprises a composition of polymericparticles and a wet strength resin dissolved in a solvent, preferablythe wet strength agent comprise an aqueous composition. Suitably, theaqueous composition has a solid content of 5-50 weight percent.

[0034] The invention further relates to a wet strength resin as abovedescribed.

[0035] The invention also relates to the use of the paper wet strengthresin and agent, as described above for the production of paper,preferably tissue paper. The use comprises addition of the resin oragent to an aqueous suspension containing cellulosic fibres. The amountof resin added to dry cellulosic fibres may be in any proportions,suitably 1-70, preferably 5-50, more preferably 15-50, and mostpreferably 25-50 kg/tonne dry cellulosic fibres. The grammage of theproduced paper suitably is lower than about 70 g/m², preferably lowerthan about 60 g/m², and most preferably lower than 40 g/m². The paperwet strength resin and agent are preferably produced as aqueousdispersions which comprise the resin, water and optionally emulsifiedparticles. The dispersion can then be added to an aqueous cellulosicsuspension to treat paper-forming cellulosic fibres. The paper wetstrength resin and agent may also be added to the produced paper andthus providing surface treatment of the paper. Further, the addition ofthe wet strength resin or agent may be added together with any otherchemical known in the art conventionally used in the production ofpaper, e.g. sizing agents, softeners, retention aids, dewatering agents,dry strength agents, charge control agents or any other conventionalchemicals, e.g. guars, carboxymethyl cellulose, polyacrylamide,polystyrene. Further, conventional fillers can be added thereto, e.g.clay, calcium carbonate, titanium dioxide, talc, aluminium silicate,calcium sulphate, calcium silicate or others described in WO 97/37080.Further, the wet strength agent may be added to the cellulosicfibre-containing suspension in any proportion. Before the wet strengthresin or agent are added to an aqueous cellulosic suspension, theaqueous dispersion containing the resin or agent may be subjected toremoval of toxic by-products by means of ion exchange, electrodialysis,enzymatical treatment, filtration, steam stripping or the like in orderhot to add any toxic products, e.g. chloropropandiol, dichloropropanolto the cellulosic suspension. These methods are further described in forexample EP 666 242 A1, EP 510 987 A1 and WO 92/22601.

[0036] The invention further relates to a process for the production ofpaper, preferably tissue paper, comprising addition of a paper wetstrength resin and/or an agent as described and exemplified herein to anaqueous cellulosic suspension. The invention also relates to paper,preferably tissue paper, comprising a wet strength resin and/or an agentas described and exemplified herein. By tissue paper is generally meantitems such as facial, hand, and toilet tissues used as a personal careproduct which comprises two key elements: a substrate formed of a planarmaterial commonly known as tissue paper and an emollient which iscarried by the substrate. In this context, tissue paper also comprisesapplications for domestic and industrial use, such as wiping of objectsby means of kitchen rolls or the like. Tissue paper is generallyproduced from an aqueous suspension of cellulosic fibres, to whichsuspension wet strength agents have been added. The cellulosefibre-containing aqueous suspension is thereafter dewatered, suitably toa consistency of between about 7% and 25% water, suitably by means ofvacuum dewatering and pressing operations such as opposing mechanicalmembers, e.g. cylindrical rolls, to obtain a wet cellulosefibre-containing web. The dewatered web is further pressed duringtransfer and dried suitably by a stream drum apparatus known in the artas a Yankee dryer. Vaccum may also be applied to the web as well asmultiple Yankee dryer drums, whereby additional pressing is optionallyincurred between the drums, thereby forming tissue paper structures. Thesubstrate can either consist of a single ply of tissue paper or it cancomprise a laminate of two or more plies of tissue paper. In eitherevent, since the substrate is formed of tissue paper, it is contemplatedthat it will be relatively thin in comparison to its dimensions in itsmajor plane. As a relatively thin planar material, the substrate willhave two major surfaced. Four important physical attributes of tissuepapers are their strength, their softness, their absorbency,particularly for aqueous systems, and their lint resistance,particularly their lint resistance when wet, as further described inWO95/01478. Production methods for producing tissue paper are furtherdescribed in WO95/01478, hereby incorporated by reference. More specificapplications or uses of tissue paper include receiving and containingdischarges from the human body, which can be used to wipe portions ofthe human body to remove substances therefrom, and which can be used todeposit materials thereon. The inventional paper wet strength resin oragent suitably has hydrophobic side-chains containing 6-40 carbon atoms,preferably 8-40 carbon atoms. Hydrophobic side-chains may be derivedfrom (meth)acrylates, alkenyl(meth)acrylate, alkyl(meth)acrylamides,esters, ethers, diazo compounds, carboxylic acids, acid anhydrides,epoxides, alkyl sulphonates, alkyl sulphates and mixtures or derivativesthereof containing a hydrophobic group, preferably fromalkyl(meth)acrylates, alkyl(meth)acrylamides, alkyl sulphonates, alkylsulphates, diazo compounds, ethers, or epoxides or mixtures thereof, andmost preferably from alkyl(meth)acrylates, alkyl(meth)acrylamides ormixtures thereof. Other suitable hydrophobic side-chains may be derivedfrom substituted succinic anhydrides containing a group selected fromalkyl, alkenyl, aralkyl, or aralkenyl, and ketone dimers or multimers.Further examples of suitable hydrophobic side-chains may be derived fromthe hydrophobic compounds disclosed in e.g. WO98/39376, U.S. Pat. No.9,922,243, hereby incorporated by reference. The grammage of theproduced tissue paper suitably is lower than about 70 g/m², preferablylower than about 60 g/m², and most preferably lower than 40 g/m². Theamount of resin or agent added to a certain amount of dry cellulosicfibres may be in any proportions, suitably from about 1 to about 70kg/tonne dry cellulosic fibres, preferably from about 5 to about 50,more preferably from about 15 to about 50, and most preferably fromabout 25 to about 50 kg/tonne dry cellulosic fibres. According to onepreferred embodiment, a further dry strength agent is added incombination with the inventional paper wet strength resin or agent, e.g.starch, guar, carboxymethylcellulose (CMC) or a synthetic dry strengthagent such as anionic or amphoteric polyacrylamides, even though theaddition level of the inventional paper wet strength resin or agent tothe aqueous cellulosic suspension is from about 5 to about 50 kg/tonnedry cellulosic fibres. In order to adjust a suitable dry strength of theproduced tissue paper, a person skilled in the art can select a suitablehydrophobic wet strength resin or agent to obtain a desirable tissuepaper, whereas the wet strength of the tissue paper can be controlled byadding an appropriate amount of resin or agent to the aqueoussuspension. A tissue paper having a high relative wet strength canthereby easily be achieved.

[0037] The invention further refers to a tissue paper containing a wetstrength agent in the form of a nitrogen-containing polymer havinghydrophobic side-chain substituents as disclosed above and which tissuepaper before converting has a relative wet strength value (RWS) of atleast 45%.

[0038] The relative wet strength value (RWS) is defined as: RWS(%)=WS/DS wherein WS=wet strength index and DS=dry strength index.

[0039] The tissue paper according to the invention contains an amount ofwet strength agent from 1 to 4% by weight, preferably between, 1.2 to 3%by weight.

[0040] In preferred embodiments the tissue paper before converting has arelative wet strength value (RWS) of at least 47% % and more preferablyat least 50%. Preferably the relative wet strength value is up to 60%.

[0041] The tissue paper may further contain an amount of a dry strengthagent. Examples of preferred dry strength agents are carboxy alkylpolysaccharides, for example carboxy alkyl cellulose.

[0042] The tissue paper should preferably have a dry strength index ofat least 5 Nm/g and no more than 10 Nm/g.

[0043] A tissue paper having the above characteristics is wet strong andsoft and it also has a sufficient dry strength for handling inconverting operations

[0044] The invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the gist and scope of the present invention, and allsuch modifications as would be obvious to one skilled in the art areintended to be included within the scope of the claims. While theexamples below provide more specific details of the reactions, thefollowing general principles may here be disclosed.

[0045] Today the only way of creating a high wet strength is by one ormore of the following methods:

[0046] adding as much wet strength resin as possible;

[0047] using a high amount of refining energy;

[0048] adding dry strength chemicals.

[0049] This strategy leads to a high wet strength and a high drystrength, meaning that the dry strength is only a tool for improving wetstrength. The dry strength level in tissue paper therefor usually ishigher then necessary, which leads to a stiff and coarse paper.

[0050] It has according to the invention been found that a soft and wetstrong tissue paper having a basis weight between 10 and 50 g/m², a bulkbetween 2 and 8 g/m³ and a wet strength index of at least 1 Nm/g, can beobtained by the fact that the tissue paper contains a wet strength agentin the form of a nitrogen-containing polymer having hydrophobicside-chain substituents as disclosed above and by balancing the wetstrength index and dry strength index of the paper so as to provide arelative wet strength index (RWS) of at least 45%, preferably at least47 and more preferably at least 50%. The RWS value refers to the valuebefore converting operations, such as embossing, gluing, laminationetc., since these may effect the properties of the paper.

[0051] Wet strength agents which increase the wet strength of tissuepaper without increasing the dry strength are wet strength agents asdisclosed above.

[0052] The addition to tissue paper of wet strength agents in the formof the nitrogen-containing polymers having saturated or unsaturatedhydrophobic side-chain substituents will cause an increase of the wetstrength index of the paper, due to the formation of covalent bondsbetween the cellulose molecules and the nitrogen-containing polymers.This is the same mechanism as in conventional wet strength agents. Theaddition of conventional wet strength agents will also increase theamount of hydrogen bond sites resulting in an increased dry strength.However due to the presence of the hydrophobic side-chain substituentsin the wet strength agents used in the present invention the hydrogenbond sites will decrease. Since the dry strength of paper to a highdegree is dependant on the amount of hydrogen bonds, a decrease of thehydrogen bonds will also result in a decrease of the dry strength of thepaper.

[0053] Thus an increase of the wet strength index of the tissue paper isobtained, without increasing the dry strength index by adding the wetstrength agents in the form of the nitrogen-containing polymers havinghydrophobic side-chain substituents. The hydrophobic side chains may besaturated or unsaturated. In fact a decrease of the dry strength indexmay even be obtained.

[0054] Evaluation of the dry strength of the tissue paper was performedaccording to the standard method SCAN P 44:81. The wet strength wastested according to SCAN P 58:86. The tensile strength is presented asindex values or geometrical mean values of machine and cross directionaccording to:

{square root}{square root over (MD×CD)}/grammage Nm/g

[0055] The tissue paper should preferably contain the wet strength agentin an amount from 1 to 4% by weight, more preferably from 1.2 to 3% byweight. These values refer to the amount of wet strength agent adheringto the fibres and measured according to the so called total nitrogenmethod. This method is based on flash combustion and is called DumasTotal Nitrogen Analysis. The measuring instrument used is Carlo ErbaInstrument NA 1500 supplied by CE Termo Quest. A manual is suppliedtogether with the instrument.

[0056] The wet strength agent is preferably in the form of an aqueousdispersion and may be added to an aqueous cellulosic suspensioncontaining the papermaking cellulosic fibers. The wet strength agent mayalso be added to the produced, paper so as to provide a surfacetreatment of the paper. cellulosic fibers. The wet strength agent mayalso be added to the produced paper so as to provide a surface treatmentof the paper.

[0057] The tissue paper should preferably have a dry strength index ofat least 5 Nm/g in order to be handled in converting processes such asrolling, unrolling, cutting, embossing, lamination etc. An addition of adry strength agent, such as a carboxy alkyl polysaccharide, for examplecarboxy alkyl cellulose, especially carboxy methyl cellulose (CMC), maybe necessary in order to obtain a sufficient dry strength of the tissuepaper. These dry strength agents are anionic and will contribute inadsorbing more of the cationic wet strength agent to the fibres. The drystrength index should preferably be no more than 10 Nm/g in order tokeep the softness as high as possible.

[0058] The tissue paper may also contain further additives such assoftening agents, absorption enhancing agents, fillers etc.

[0059] The following examples will further illustrate how the describedinvention may be performed without limiting the scope of it.

EXAMPLE 1

[0060] Reaction of a polyaminoamide (hereinafter also called PAIM)(produced from adipic acid and diethylene triamine) with a hydrophobiccompound (vinylog addition); 240 g (0.60 amino-mol equivalents) PAIM(53% solution in water) and 27.3 g (0.15 mol) 2-ethylhexyl acrylate(2-EHAc): were heated for 6 h and 30 min at 80° C. Subsequently, 176 gof water was added and the solution was cooled down to room temperature.Conversion of acrylate was: 99.7%.

[0061] 307 g of the above hydrophobised PAIM solution was reacted with30 ml epichlorohydrine (ECH) at 6° C. for 6 min. Subsequently, thetemperature was increased until 20° C. was reached. The temperature wasthen increased until 50° C. and a viscosity of 120 mPa s was reachedwhereupon 155 ml of water was added and the temperature was adjusted to65° C. to let the viscosity reach 120 mPa s. The reaction was finalisedby adding 11 ml of sulfuric acid (50%) adjusting the pH to 3.5.

[0062] Emulsion polymerisation: The ratio resin to styrene was 1:2.

[0063] A solution of 47 g of the above produced wet strength resin, 104g water and 1.5 ml defoamer (10% solution in water) was purged withnitrogen. The temperature was then increased to 50° C. whereupon 0.5 gWako VA 044 and 1 ml styrene were added to the solution. 10 min later,additional styrene was added (total amount: 25 g). After 5 h at 50° C.,the temperature was increased to 70° C. at which temperature thesolution was kept for an hour.

EXAMPLE 2

[0064] Reaction of Polyaminoamide (PAIM) with a 2-ethylhexylacrylate(2-EHAc) (vinylog addition): 82 g (0.20 amino-mol equivalent) PAIM (52%solution in water), 1.84 g (0.01 mol) 2-ethylhexyl acrylate (2EHAc) and43 g of water were heated for 2 h at 80° C. Conversion of acrylate:98.9%.

[0065] 15,4 ml epichlorohydrine (ECH) was added to 125 g of the abovehydrophobised PAIM solution at 6° C. for 6 min. Subsequently, thetemperature was increased until 20° C. was reached. The temperature wasthen increased to 65° C. and a viscosity of 120 mPa s was reachedwhereupon 86 ml of water was added. The temperature was raised to 65° C.and kept at 65° C. until the viscosity reached 120 mPa s. The reactionwas finalised by addition of 11 ml sulfuric acid (50%) adjusting the pHto 3.5.

[0066] Emulsion polymerisation:. The resin/styrene ratio was 1:0.5. Asolution of 88.5 g of the above wet strength resin, 92 g water and 1.5ml defoamer (10% solution in water) was purged with nitrogen. Thetemperature was increased to 45° C. 0.04 g Wako VA 044 and 2 ml styrenewere added whereafter the temperature was raised to 50° C. After 10minutes, additional styrene was added (total amount: 12 g). After 3 h at50° C., the reaction mixture was cooled down to room temperature.

EXAMPLE 3

[0067] 260 g (0.65 amino mol equivalent) PAIM (53% solution in water)(Polyaminoamide, PAIM) and 25% 41.0 g (0.16 mol) dodecyl acrylate(vinylog addition) were heated for 4 h 30 min at 80° C. Subsequently,211 g water was added whereafter the mixture was cooled down to roomtemperature.

[0068] 302 g of the above hydrophobised PAIM was then reacted with 30 ml(0.20 mol) epichlorohydrine (ECH)at 8° C. for 4 min. Subsequently, thetemperature was increased until 20° C. was reached. The temperature wasthen increased until 50° C. and a viscosity of 120 mPa s was reached.185 ml water was then added and the temperature was raised to 65° C. andkept at that temperature until the viscosity reached 120 mPa s. Thereaction was finalised by addition of 10 ml sulfuric acid (50%)adjusting the pH to 3.5.

[0069] Emulsion polymerisation:. The resin/styrene ratio was 1:1. Asolution of 75.0 g of the above wet strength resin, 100 ml water and 1ml defoamer (10% solution in water) was purged with nitrogen. Thetemperature was increased to 50° C. whereupon 30 mg Wako VA 044 and 1 mlstyrene were added. After 10 minutes additional styrene was added (totalamount: 20.5 g). After 5 h at 50° C., the temperature was increased to70° C. and set at that temperature for one hour.

EXAMPLE 4

[0070] In the emulsion polymerisation, butyl acrylate was used insteadof styrene. A solution of 75.0 g of the wet strength resin of example 3(13% solids), and 1.5 g defoamer (10% solution in water) was purged withnitrogen. The temperature was increased to 45° C. 0.03 g Wako VA 044 and2 ml butyl acrylate were then added whereupon the temperature wasincreased to 50° C. After ten minutes, styrene was added (total amount:14.2 ml). After 2 h 50 min at 50° C., the temperature was increased to70° C. which temperature was kept for one hour.

EXAMPLE 5

[0071] 25% 2-ethylhexyl acrylate was used to hydrophobise PAIM. Emulsionpolymerisation:. A solution of 121 g of the wet strength resin ofexample 1 (solids 28%), 131 a water and 1 ml defoamer (10% solution inwater) was purged with nitrogen. The temperature was increased to 45° C.0.04 g Wako VA 044 and 2 ml of a monomer mixture (styrene: 1,6-hexandioldiacrylate=0.375: 0.125) were added whereupon the temperature was raisedto 50° C. in 10 min. Subsequently, the monomer mixture was added (totalamount: 17 g). After 3 h at 50° C. the reaction mixture was cooled downto room temperature.

EXAMPLE 6

[0072] 25% 2-ethylhexyl acrylate was used to hydrophobise PAIM. Amonomer mixture of styrene with t-butyl acrylate (0.45:0.05) was used,Emulsion polymerisation: A solution of 121 g of the wet strength resinof example 1 (solids 28%), 131 g of water and 1 ml defoamer (10%solution in water) was purged with nitrogen. The temperature wasincreased to 45° C. 0.04 g Wako VA 044 and 2 ml of a monomer mixture(styrene: t-butyl acrylate=0.45:0.05) were then added and thetemperature was raised to 50° C. in 10 min. Subsequently, the monomermixture was added (total amount: 17.0 g). After 3 h at 50° C. thereaction mixture was cooled down to room temperature.

EXAMPLE 7

[0073] 630 g (1.67 amino-mol equivalent) PAIM (56% solution in water)and 12% (0.2 mol) dodecyl acrylate (vinylog addition) were heated for 6h at 80° C. Subsequently, 326 g water was added whereafter the mixturewas cooled down to room temperature. Conversion of the acrylate was>99%.

[0074] 1005 g of the above hydrophobised PAIM was then reacted with 155g (1.68 mol) epichlorohydrine (ECH) at 8° C. for 4 min. Subsequently,the temperature was increased until 20° C. was reached. The temperaturewas then increased until 50° C. and a viscosity of 120 mPa s was reached287 ml water was then added and the temperature was raised to 65° C. andkept at that temperature until the viscosity reached 100 mPa S. Thereaction was finalised by addition of 50 ml sulfuric acid (50%) and 513ml water adjusting the pH to 3.5.

EXAMPLE 8

[0075] 308.5 g (0.81 amino-mol equivalent) PAIM (55% solution containingin water) and 15% (0.12 mol) benzyl chloride (alkylation reaction) wereheated for 6 h at 60° C. Subsequently, the mixture was cooled down toroom temperature.

[0076] 125.5 g of the above hydrophobised PAIM was then reacted with17.7 g (0.19 mol) epichlorohydrine (ECH) at 6° C. for 4 min.Subsequently, the temperature was increased until 20° C. was reached.The temperature was then increased until 50° C. and a viscosity of 120mPa s was reached. 33 ml water was then added and the temperature wasraised to 65° C. and kept at that temperature until the viscosityreached 100 mPa s. The reaction was finalised by addition of 6 mlsulfuric acid (50%) adjusting the pH to 3.5.

EXAMPLE 9

[0077] 350 g (0.91 amino-mol equivalent) PAIM (55% solution in water)and 15% (0.14 mol) 2-ethylhexyl glycidyl ether (alkylation reaction)were heated for 7.5 h at 60° C. Subsequently, the mixture was cooleddown to room temperature.

[0078] 130.4 g of the above hydrophobised PAIM was then reacted with17.7 g (0.19 mol) epichlorohydrine (ECH) at 6° C. for 4 min.Subsequently, the temperature was increased until 20° C. was reached.The temperature was then increased until 50° C. and a viscosity of 120mPa s was reached. 33 ml water was then added and the temperature wasraised to 65° C. and kept at that temperature until the viscosityreached 100 mPa s. The reaction was finalised by addition of 5.7 mlsulfuric acid (50%) adjusting the pH to 3.5.

EXAMPLE 10

[0079] 274 g (0.71 amino-mol equivalent) PAIM (55% solution in water)and 3.8% (0.027 mol) alkyl ketene dimer (C18-chains) were heated for 6 hat 60° C. Subsequently, the mixture was cooled down to room temperature.

[0080] 127.2 g of the above hydrophobised PAIM was then reacted with17.7 g (0.19 mol) epichlorohydrine (ECH) at 6° C. for 4 min.Subsequently, the temperature was increased until 20° C. was reached.The temperature was then increased until 50° C. and a viscosity of 120mPa s was reached. 33 ml water was then added and the temperature wasraised to 65° C. and kept at that temperature until the viscosityreached 100 mPa s. The reaction was finalised by addition of 5.7 mlsulfuric acid (50%) adjusting the pH to 3.5.

EXAMPLE 11

[0081] 274 g (0.71 amino-mol equivalent) PAIM (55% solution in water)and 5% (0.036 mol) alkenyl succinic anhydride (C18-chains) were heatedfor 6 h at 60° C. Subsequently, the mixture was cooled down to roomtemperature.

[0082] 124.3 g of the above hydrophobised PAIM was then reacted with17.7 g (0.19 mol) epichlorohydrine (ECH) at 6° C. for 4 min.Subsequently, the temperature was increased until 20° C. was reached.The temperature was then increased until 50° C. and a viscosity of 120mPa s was reached. 33 ml water was then added and the temperature wasraised to 65° C. and kept at that temperature until the viscosityreached 100 mPa s. The reaction was finalised by addition of 5.7 mlsulfuric acid (50%) adjusting the pH to 3.5.

EXAMPLE 12

[0083] 185.4 g (0.48 amino-mol equivalent) PAIM (54% solution in water)and 10% (0.048 mol) hexanediol diacrylate (90%) were heated for 4.5 h at80° C. Subsequently, the mixture was cooled down to room temperature.Conversion of acrylate: >99%.

[0084] 124.0 g of the above hydrophobised PAIM was then reacted with17.7 g (0.19 mol) epichlorohydrine (ECH) at 6° C. for 4 min.Subsequently, the temperature was increased until 20° C. was reached.The temperature was then increased until 50° C. and a viscosity of 120mPa s was reached. 33 ml water was then added and the temperature wasraised to 65° C. and kept at that temperature until the viscosityreached 100 mPa s. The reaction was finalised by addition of 6.7 mlsulfuric acid (50%) adjusting the pH to 3.5.

[0085] Application Testing

[0086] Paper sheets were prepared on the dynamic sheet former“Formette”. The furnish consisted of 35% CTMP and 65% TCF refined to 25°SR. The paper was artificially cured 10 min at 105° C. beforeconditioning the paper according to DIN 5312. Tensile testing was doneas described in DIN 53112. For wet tensile testing the paper was soaked60 min at room temperature. For comparison reasons, data on paperprepared by using a conventional polyaminoamide epichlorohydrin resinhas also been given. It is to be noted that the paper sheets belowappearing in tables 1-4 have been tasted at three different occasionsusing different addition levels of the wet strength agent used. Inexample 1-6, 20 kg wet strength agent was added/tonne cellulosic fibres.The grammage was 55 g/m². In examples 7-12. the grammage was 30 g/m² andthe addition levels of wet strength resin were 15, 20 and 30 kg/tonnecellulosic fibres. As a consequence thereof, observed values of relativestrength vary between each occasion. A reference resin, i.e. aconventional resin, has therefore been measured at each occasion asappears from the tables 1-4 below. As can be seen from the examples, thewet strength resins and agents show superior effect in view of theconventional resin used as reference at the same addition levels.

[0087] Wet-strong Tissue Paper Tests

[0088] Four different wet strength agents were produced according toExample 7 above. The wet strength agents were designated A, B, C and D,of which D were produced exactly as disclosed in Example 7, while theother three were produced as disclosed but with the modification thatvarying amounts of dodecyl acrylate were used.

EXAMPLE 13

[0089] Tissue paper having a basis weight of about 25 g/m² and a bulk at2 kPa of about 6-6.5 g/m³ was produced on a full scale paper machine.The, pulp used was a mixture of 70% by weight CTMP(chemothermomechanical pulp) and 30% by weight sulphate softwood pulp.Varying amounts of the wet strength agents A and B were added. Theresults obtained are presented in Table 5 below. with a dry strengthagent in the form of CMC (carboxy methyl cellulose). The resultsobtained are presented in Table 6 below.

EXAMPLE 15

[0090] Tissue paper having a basis weight of about 26.5 g/m² and a bulkat 2 kPa of about 2.5 g/m³ was produced on a full scale paper machine.The pulp used was recycled newsprint mixed. The wet strength agent D wasused in combination with a dry strength agent in the form of CMC(carboxy methyl cellulose). The results obtained are presented in Table7 below.

[0091] These tests showed that it is possible to achieve relative wetstrength values (RWS) of 45% and higher by using relatively high amountsof the wet strength agents containing hydrophobic side-chainsubstituents. The wet strength agent may advantageously be combined withan anionic dry strength agent in order to adsorb more of the cationicwet strength agent to the fibres. TABLE 1 Dry tensile Wet tensileRelative wet Sample index in Nm/g index in Nm/g strength in %Conventional resin 49 13 27 Example 1 37 14 37 Wet Strength EmulsionExample 2 51 15 30 Wet Strength Emulsion Example 3 37 12 32 Wet StrengthResin Example 3 37 13 34 Wet Strength Emulsion Example 4 33 12 36 WetStrength Emulsion Example 5 35 11 31 Wet Strength Emulsion Example 6 3712 33 Wet Strength Emulsion

[0092] TABLE 2 Sample Dry tensile index Wet tensile index Relative wet15 kg/ton of paper in Nm/g in Nm/g strength in % Conventional resin 40.39.7 23.9 Example 7 31.6 9.3 29.5 Wet strength resin Example 8 38.3 11.028.7 Wet strength resin Example 9 33.6 9.0 26.7 Wet strength resinExample 10 40.3 10.7 26.6 Wet strength resin Example 11 35.3 10.7 30.2Wet strength resin Example 12 38.6 10.3 26.7 Wet strength resin

[0093] TABLE 3 Sample Dry tensile index Wet tensile index Relative wet20 kg/ton of paper in Nm/g in Nm/g strength in % Conventional resin 41.610.3 24.8 Example 7 31.6 9.3 29.5 Wet strength resin Example 8 38.0 10.828.5 Wet strength resin Example 9 35.0 10.0 28.6 Wet strength resinExample 10 39.3 11.0 28.0 Wet strength resin Example 11 35.0 11. 31.4Wet strength resin Example 12 37.3 10.7 28.6 Wet strength resin

[0094] TABLE 4 Sample Dry tensile index Wet tensile index Relative wet30 kg/ton of paper in Nm/g in Nm/g strength in % Conventional resin 40.010.7 26.7 Example 7 31.6 10.0 31.6 Wet strength resin Example 8 39.311.7 29.7 Wet strength resin Example 9 34.0 11.0 32.4 Wet strength resinExample 10 38.3 11.3 29.6 Wet strength resin Example 11 34.3 11.3 33.0Wet strength resin

[0095] TABLE 5 Added amount Dry strength Wet strength Rel wet strengthWet strength (kg/t pulp index index index agent fibers) (Nm/g) (Nm/g)RWS (%) A 7 10.2 3.4 33 A 10 10.1 4.0 40 A 15 8.7 3.8 44 A 20 7.9 4.1 52B 7 8.4 3.6 43 B 10 8.8 3.7 42 B 15 8.3 4.0 48 B 20 9.0 4.1 46

[0096] TABLE 6 Added Added amount amount CMC Dry Wet Wet Wet strength(dry strength strength strength Rel. wet strength agent (kg/t agent)(kg/t index index strength agent pulp fibers) pulp fibers) (Nm/g) (Nm/g)RWS (%) C 16 3 12.9 4.7 33 C 18 3 13.5 4.8 31 C 25 3 50

[0097] TABLE 7 Added Added amount amount CMC Dry Wet Wet Wet strength(dry strength strength strength Rel. wet strength agent (kg/t agent)(kg/t index index strength agent pulp fibers) pulp fibers) (Nm/g) (Nm/g)RWS (%) D 20 2.3 9.3 4.2 46 D 20 3.3 8.8 5.0 57 D 15 2 8.0 3.8 48 D 15 310.6 4.1 39

1. Tissue paper having a bulk between 2 and 8 g/m³, said tissue papercontaining an amount of a wet strength agent and having a wet strengthindex of at least 1 Nm/g, wherein the tissue paper contains a wetstrength agent in the form of a nitrogen-containing polymer havinghydrophobic side-chain substituents and before converting has a relativewet strength value (RWS) of at least 45%.
 2. Tissue paper as claimed inclaim 1, wherein the nitrogen containing polymer is a polyamine or apolyaminoamide.
 3. Tissue paper as claimed in claim 2, wherein thehydrophobic side-chain substituents comprise a hydrophobic chain having6-40 carbons.
 4. Tissue paper as claimed in any of claims 1-3, whereinit contains a wet strength agent in an amount from 0.7 to 4% by weight,preferably from 1 to 3% by weight.
 5. Tissue paper as claimed in any ofclaims 1-4, wherein it before converting has a relative wet strengthvalue (RWS) of at least 47% and more preferably at least 50%.
 6. Tissuepaper as claimed in any of claims 1-5, wherein it before converting hasa relative wet strength value (RWS) of up to 60%.
 7. Tissue paper asclaimed in any of claims 1-6, wherein it also contains an amount of adry strength agent.
 8. Tissue paper as claimed in claim 7, wherein thedry strength agent is a carboxy alkyl polysaccharide.
 9. Tissue paper asclaimed in claim 8, wherein the carboxy alkyl polysaccharide is acarboxy alkyl cellulose.
 10. Tissue paper as claimed in any of claims1-9, wherein it has a dry strength index of at least 5 Nm/g.
 11. Tissuepaper as claimed in claim 10, wherein it has a dry strength index of nomore than 10 Nm/g.